Mirrored UV map is breaking up textures per-face
It's becouse faces in that place are "rotated". Let's i describe it in image:



your vertex number 1 is on position of vertex 3, n.2 on position of 1, n.3 on pos. 2.

so it look like all vertex on right place, but this places are wrong

Look at my another example. As you see all faces are in right place, but selected face is rotated at 90 degrees. Same thing in your model, but many of faces are rotated.



So if you have original model - try to transfer it UV map using Data Transfer modifier